1. Field of the Invention
The present invention relates to hearing aids. The invention further relates to methods of utilizing gain-limitation in hearing aids. The invention, more particularly relates to hearing aids incorporating multiple microphones that are adapted to interpolate a maximum gain limit in dependency of the mixing ratio of the microphone signals. The invention still more particularly, relates to hearing aids further incorporating feedback cancellation in order to reduce disturbances due to acoustic feedback, and respective methods thereof.
2. The Prior Art
It is a widely known problem in hearing aid design to adjust the maximum possible amount of gain with which an acoustic input signal may be amplified to produce a hearing loss compensation signal without the appearance of artifacts cased by acoustic feedback or other acoustic disturbances. This is in particular a problem in hearing aids that incorporate multiple microphone branches each having a microphone providing a feedback path. Therefore, a gain safety margin is generally required in order to avoid that the feedback loop approaches the border of stability, the point of generating undesired and annoying sounds.
WO-A-94/09604 discloses a hearing aid with digital, electronic compensation for acoustic feedback which comprises a compensation circuit. The circuit monitors the loop gain and regulates the hearing aid amplification so that the loop gain is less than a constant K. An adaptive filter operates to minimize the correlation between input and output from the hearing aid and may be used to give a measure of the attenuation in the acoustic feedback path by deriving gain, and possibly also phase, characteristics from a feedback cancellation filter.
WO-A-02/25996 discloses a hearing aid with an adaptive filter for suppression of acoustic feedback. The adaptive filter may be used as an independent measuring system to estimate the acoustic feedback signal without distortion of the processed acoustic input signal.
These data may be used to determine loop gain and then set an upper limit on the applicable gain that may be used in each of multiple evaluated frequency bands.
Also, U.S. Pat. No. 6,498,858 discloses how feedback cancellation may be applied to a system with two omni-directional microphones.
Neither of these publications discloses, however, how maximum gain limit values can be determined in multi-microphone systems.
WO-A-99/26453 discloses a feedback compensation system for a hearing aid with two microphones and directional processing, wherein each microphone signal is independently feedback compensated before processing in a directional controller. Independently compensating each microphone signal before directional processing requires extensive processing and carries a risk that an imperfect compensation of the feedback signals will result in a residual feedback signal component, which may interfere with the function of the directional controller.
Thus, there is a need for improved hearing aids as well as improved techniques for utilizing gain-limitation in multi-microphone hearing aids.